1. Field of the Invention
This invention relates to devices that are intended to transfer heat from one fluid to another, particularly such devices that are suitable for use in hot gas engines.
2. Description of the Prior Art
Heat is frequently needed to be added to, or extracted from a stream of moving fluid. This is commonly done by heat exchangers which transfer heat between two streams of moving fluid. A common example is in Stirling or hot-gas engines which require three heat exchangers called a heater, cooler and regenerator. The regenerator is a special type of heat exchanger with heat storage capability, and will not be discussed further. Heaters and coolers have design requirements which apply to other types of heat exchangers and include: High efficiency of transfer; controlled resistance to flow of fluid; capability to resist high temperatures and pressures; and, low cost of construction.
Several types of heater exchangers have been developed to accomplish the above design criteria. The methods can be characterized as cylinder heating, finned heat exchangers, tubular heat exchangers, baffle arrangements, and combinations of the above. Additionally, heat pipes have been used with each of the above structures. Construction of a suitable heater has, nonetheless, been one of the primary obstacles to widespread adoption of Stirling or hot gas engines.
The simplest method used is aiming a burner or other heat source at the cylinder of the engine. This method is used with small engines for demonstration purposes. This method has the advantage of low cost and simplicity, but the efficiency of heat transfer is low and the method cannot be scaled up for use in larger engines.
A variation of the above method is accomplished by adding fins to the cylinder to make a finned heat exchanger. This method can also be accomplished by adding a separate finned heat exchanger. The fins add to the efficiency of heat transfer by increasing the area exposed to heat. On the negative side, the addition of fins adds to the cost of construction as they must be welded or brazed to the cylinder or heat exchanger to operate at high temperature. The fins do not affect the pressure handling capability of the device where there is still a trade-off between thin walls for efficient transfer and thick walls for pressure resistance. As a result of the above factors, fins are generally only used in small engines.
The next most common approach has been the addition of a tubular heat exchanger. A tubular heat exchanger usually consists of a series of bent tubes running between the cylinder and regenerator. The tubes are arranged to be exposed to a burner or other heat source. To achieve high thermal efficiency, the tubes should have thin walls. Thin walled tubing, however, is limited in its pressure handling capability. Additionally, the numerous small tubes must be assembled and joined to the cylinder increasing the cost of construction. Nonetheless, tubular heat exchangers are the type most commonly used for larger engines.
Combinations of the above methods have also been used, such as fined tubes. Generally the additional cost of construction outweighs the advantages. One addition frequently used is a heat pipe to convey heat from a small burner to a large heat exchanger structure. A heat pipe is a cavity filled with a vaporizable liquid, such as sodium metal and an inert gas. One portion of the cavity is heated, and the liquid vaporized. This is called the evaporation area. The load is located at another portion of the cavity called the condensation area. At the condensation area, the liquid condenses, transferring its heat of evaporation to the load. The result is a conveying of heat with low loss. The use of heat pipes, however, cannot increase the pressure or temperature capabilities.
Finally, baffle arrangements have been tried alone, or in combination with the other methods. The baffles can increase thermal efficiency, but at an increase in construction cost. In summary, no method has been found that provides high thermal efficiency at low cost with the ability to be exposed to constant high temperatures and pressures at controlled flow resistance.